The effect of low pH on autotrophic ammonia oxidation was to be investigated. Autotrophic ammonia oxidisers were successfully isolated from soils of low pH, from sites around Scotland, in an attempt to determine if acid tolerant or acidiphilic strains were responsible for nitrification in these soils. No acid tolerant bacteria were isolated and adaptation, of nitrifiers, to low pH was not found to have occurred during the maintenance of agricultural soil plots at low pH. Carbonate was found to be limiting at low pH, if sodium carbonate, alone, was used to adjust the pH of the medium. The pH minima for ammonia oxidation was not affected by additional carbonate. Recently isolated nitrifying bacteria, grown in liquid culture, were found to produce large amounts of exopolysaccharides at stationary phase, causing cell aggregation. Evidence suggested that this material offered protection against desiccation. Continuous flow columns were used to study surface attached N. europaea at low pH. It was demonstrated that surface attachment allowed nitrification to occur at 1.3 pH units lower than in liquid batch culture. This system also demonstrated a requirement for additional carbonate in medium of low pH. Evidence was found to indicate that ammonium is transported into the cell and that NH3 is not a limiting factor due to low pH. A nitrifying biofilm showed that attachment within a polysaccharide matrix offered significant persistence in a low pH environment and that activity occurred at a value lower than in liquid batch culture. The sensitivity of N. europaea to inhibition by PEX was found to increase in liquid batch culture. Continuous flow soil columns showed nitrapyrin to be more inhibitory at low pH. Nitrification occurred in columns at a pH value lower than in liquid batch culture. This culture system suggested that the bacteria were in a different physiological state than when grown in batch culture. Several strains of ammonia oxidisers, isolated from acid soils, were shown to possess a urease enzyme. A Nitrosospira sp exhibited limited growth on urea at pH 5.5.